The present invention relates to a radiation detector used in a sectional radiographic apparatus such as a computerized tomographic (hereafter abbreviated as CT) apparatus.
In a CT apparatus, as shown in FIG. 1, a fan-shaped distribution of X-ray beam 12 is provided by an X-ray source 10. A radiation detector 14, comprising a plurality of detecting elements 16 arranged for detecting the intensity of the X-ray beam 12, faces the X-ray source 10 with a subject body 18 between them. While maintaining this positional relation, the X-ray source 10 and the radiation detector 14 are rotated around the subject body 18. In this state, the intensity of the X-ray beam 12 transmitted through the subject body 18 in various directions is detected. Detection data obtained in this manner are analyzed by a computer, and X-ray absorption factors for plane sections of the subject body 18 taken along those different directions are calculated. The cross sections of the subject body 18 are given in gradations based on the X-ray absorption factors, thus forming a sectional image. In this CT apparatus, the plane sections of the subject body 18 can be analyzed in as many as 2,000 gradations, depending on the composition of the subject body 18. Thus, a clear sectional image can be obtained for any tissue ranging from soft to hard tissue.
In the radiation detector 14, as shown in FIGS. 1 to 3, electrodes, as the detecting elements 16, are spacially arranged from one another in a housing 20. High-pressure Xe gas is sealed in the housing 20. When the X-ray beam 12 is projected into the interelectrode space on the extension of the X-ray beam 12 through an X-ray window 22 of the housing 20, the Xe gas existing in the space is ionized to produce Xe.sup.+ ions and electrons. The Xe.sup.+ ions and electrons are detected as ionization currents by the electrodes, which are integrated for a predetermined time. The integrated currents are discharged by a discharge circuit having a predetermined time constant. The intensity of X-ray beam can be calculated from the value of the discharge time.
The performance of a radiation detector is judged by its sensitivity and resolution (space resolution and density resolution). The quality of an image reconstructed on a CT apparatus is influenced by the performance of the radiation detector. The sensitivity of the radiation detector is given as the product (atm.cm; hereafter referred to as PL value) of the pressure of gas sealed in the radiation detector and the depth of electrodes. Usually, the sensitivity is in the vicinity of 60 atm.cm. In this case, the coefficient of energy absorption ranges from 40 to 60%. The space resolution of the radiation detector, which depends on the arrangement pitch of the electrodes and focus spot size of X-ray tube, is such that the detector can normally discriminate a substance with a diameter of 0.5 to 0.6 mm. The density resolution is related to the capability of distinguishing substances with small differences in density in a subject body. If the radiation detector is higher in density resolution, then it can discriminate smaller differences in density in proportion. The density resolution depends on the amount of low-energy photons which are transmitted through the X-ray window of the radiation detector to reach the interelectrode space therein. This is so because definite discrimination of white and gray matter in the subject body requires detection of the low-energy photons, since the difference in the coefficient of X-ray absorption between the white and gray matter is increased in proportion if the energy of the photons is lower.
The housing 20 of the radiation detector 14 is provided with the X-ray window 22 for the incidence of X-ray beams, which is thinner than any other portion of the housing 20. Also, the housing 20, including the X-ray window 22, is made of aluminium. However, the X-ray window 22 cannot avoid absorption of low-energy photons, failing to provide satisfactory density resolution.
A radiation detector using a carbon-fibered structure as its X-ray window is disclosed in U.S. Pat. No. 4,260,891. In this radiation detector, a carbon-fibered plate is sandwiched between a pressure vessel and a clamping lid. Therefore, the carbon-fibered plate needs to serve both as the X-ray window and as a sealing gasket between the clamping lid and the pressure vessel. Thus, this X-ray window is flat in shape. In a radiation detector, the X-ray beam has, generally, fan-shaped distribution, so that the X-ray window is curved around the X-ray source 10 for improved detection accuracy. It is difficult, however, to seal gas at a high pressure of 10 to 30 atm in the housing while keeping the carbon-fibered X-ray window curved. Accordingly, any of prior art radiation detectors is low in practicality.